The present invention relates to systems and methods for determining the condition of a battery, such as a damage level of the battery.
There are several related art technologies for evaluating different aspects of the condition of a battery. For example, voltage sensing establishes a predicted voltage discharge curve for the battery with the voltage as a function of the state of charge (SOC), measures the voltage between the terminals of the battery, and compares the measured voltage to the predicted voltage discharge curve to determine the SOC of the battery. However, voltage sensing has the disadvantages that each voltage discharge curve is unique, and that the voltage discharge curve changes as the battery ages.
Another technique is known as coulomb counting, in which the remaining capacity of the battery is determined by measuring the current entering or leaving the battery, and integrating this measurement over time. The current may be measured by using a current shunt, Hall effect transducers, or giant magnetoresistance (GMR) sensors. Coulomb counting uses a fully charged battery as a reference point, and obtains the SOC by comparing the net charge flow with the charge in the fully charged battery. However, coulomb counting has the disadvantages of not directly measuring the energy in the battery, and assuming an initial value of charge in the fully charged battery based on Peukert's law.
An additional method is a cranking test, in which the voltage of the battery is measured during engine cranking and compared to threshold values. The state of health (SOH) of the battery is determined by this comparison. However, the cranking test has the disadvantage of requiring a controlled temperature.
Further, an AC conductance test measures the dynamic conductance of the battery by applying a time-varying small-amplitude AC signal to the battery and measuring the voltage response as a function of time. However, the AC conductance test has the disadvantages that repeated tests are needed at multiple frequencies, and the accuracy is diminished when testing more than one battery.
Electrochemical impedance spectroscopy (EIS) is another method for evaluating the condition of a battery. During the charge-discharge cycles, the composition of the active chemicals in the battery changes as the chemicals are converted between the charged and discharged states. This is reflected by changes to the battery's impedance. EIS estimates the battery's impedance at different AC frequencies. However, EIS has the disadvantages of requiring expensive equipment, as well as training to interpret the results.
Therefore, it would be desirable to provide a method for determining a condition of a battery, such as its damage level, that overcomes the disadvantages discussed above. In addition, it would be advantageous to provide a method that a layperson or a technician with no knowledge of battery chemistry could perform. It would also be advantageous to provide a method in which the battery does not need to be in a rested open-circuit voltage state during testing. Further, it would be advantageous to provide a method of testing a battery during or after the manufacturing process for a new motor vehicle.